Carbon-normalized copper complexation ability (strength and capacity) varies among aquatic natural organic matter (NOM) samples according to the sources and size fractions of the NOM samples. Variations of several orders of magnitude in free/complexed Cu2+ ratios are observed for samples of differing origins. Within a given aquatic NOM sample, substantial differences are also observed as a function of the "particle size' fraction of the NOM from the sample.
At a fixed organic carbon content, copper toxicities to aquatic test organisms also show considerable variation as a function of NOM source and size fraction. The hypothesis that copper toxicity is directly proportional to "free" cupric ion is examined based on these toxicity results.
Comparison are presented between riverine and lacustrine NOM (unfractionated) samples and NOM samples obtained by leaching of plant litter with subsequent aging of the leachates for a year or more. Specifically, aquatic NOM is compared to litter leachates from dominant plant species in the watershed, providing insight into geochemical alteration of NOM as far as Cu complexing abilities and toxicities are concerned.